In the manufacture of cables for use in the telephone industry, it is usually necessary to intertwist a pair of individually insulated wires to form a twisted pair. A plurality of the twisted pairs are then fed into a strander and associated together into stranded units. In twisting operations, either the wire supply or the take-up device is rotated about the axis of the advancing wires in order to impart a unidirectional twist onto the wires. One reason for twisting wire pairs is to cancel noise emanating from an external source. When equal and opposite noise signals are coupled into a twisted wire pair, the net effect is noise cancellation. Similarly, crosstalk among wire pairs within the same stranded unit can be avoided when different amounts of twist are imparted onto each twisted pair. In this regard, a number of different twisting arrangements have been devised--some more effective than others. For example, U.S. Pat. No. 4,873,393 discloses a cable that achieves extremely low crosstalk by tightly twisting the wire pairs and by varying the twist lengths among the different wire pairs in accordance with the principles of non-uniform, twist-frequency spacing.
It is desirable, although topologically impossible, to perform unidirectional twisting without rotating the wire supply or the take-up device about an axis of rotation. Unidirectional twisting generally involves the use of heavy rotating apparatus which is burdensome from a manufacturing standpoint. However, it has been found that unidirectional twisting is not necessary in most applications. Methods have been devised for forming twisted wire pairs without the need for rotating the wire supply or take-up device by periodically reversing the direction of twist imparted to an advancing group of wires. This has become known as S-Z twisting with S referring to left-hand twists and Z referring to right-hand twists. It is usually performed with apparatuses known as accumulators which have spaced-apart twister heads. Each twister head normally has one or more sheaves rotatably mounted to a head support which itself is mounted for revolution about the axis of two or more strands advancing side-by-side through the accumulator. An example is shown in U.S. Pat. No. 4,182,107 wherein a pair of strands are pulled through a strand spreader in the form of a die having two laterally spaced passageways through which the individual strands pass. The strands pass over a pair of spaced-apart twister heads through a capstan and onto a take-up device. The twister heads are simultaneously revolved about the longitudinal axis of the strands in one direction causing the strands to become twisted between the strand-spreader die and the first twister head. Simultaneously, twists of the opposite lay direction emanate from the second twister head downstream of the twister toward the take-up device.
One method for S-Z twisting uses a device known as a variable capacity, in-line accumulator--an example of which is shown in U.S. Pat. No. 3,052,079. With this type of accumulator the twister heads are moved in unison up and down an advancing line of strands while the speed of advance of the strands, and the speed of revolution of the twister heads thereabout, are both maintained constant. Other examples of S-Z twisting are shown in U.S. Pat. Nos. 3,373,550 and 3,782,092. And while the above examples avoid the need for rotating the strand supply or the take-up device, they still require a substantial amount of apparatus and the span between twist reversals is relatively short. Twist reversals generally require that the wire pair be gripped at the reversal point to avoid unraveling,--an added complexity when short twist spans are used.
What is needed, and what seemingly is not available in the art, is a method for twisting strand materials without rotating the strand supply or the take-up device, and which delivers a longer span of unidirectional twist.